He can feel it immediately when the boy finally ends up saying yes. A buzz spreads through his grace.
The boy is a surprisingly good fit. It's easy to find the place to settle inside him, to take control. He stretches the fingers. Much better than John Winchester. He can feel the sparks of the boy's soul and unlike his father's, they aren't irritating buzz against his grace but electrifies it instead.
Then he notices the smears of blood on the hand. That won't do, not at all. One look and they disappear. Better.
Not nearly as bad as he thought it would be.